The present invention concerns a water comprising two driving barrels each containing a spring and ensuring an independent period of operation of less than 72 hours.
The precision of a watch strictly depends on the regulating power of the spring balance wheel oscillator, a conception which expresses the invariability of the period of a balance wheel under the effect of influences such as temperature, variation in the torque on the escapement wheel, accelerations and shock. The regulating capacity is proportional to the moment of inertia, the square of the amplitude, and the cube of the frequency of the balance wheel. This latter factor plays the most important part and, in particular, a "high regulating power" is referred to when the balance wheel oscillates with a frequency greater than 3.4 Hz. In order to maintain the movement of an oscillator of this type, high driving energy is necessary, too high to be provided by a spring of the conventional barrel, the volume of which is limited by the size of the movement.
Hence the idea of replacing one driving barrel by two barrels, providing the same energy, but more easily disposed rationally in a cage.
Let us consider a barrel spring capable of storing potential energy E during re-winding and of returning it in kinetic form, driving the gear train mechanism and the oscillator; the driving barrel then turns with an angular velocity w, providing a torque M (mean value), during a time T corresponding to the independent operation of the watch, namely EQU E=M.multidot.W.multidot.T (a)
If two barrels are coupled in any manner, and the springs of these are assumed to be identical, to simplify the reasoning, it is obvious that the energy 2E is then available. The equation (a) may be checked in a variety of ways, for example: ##EQU1##
With solution (b), the available energy is exploited to obtain autonomous operation for time 2T as, for example, in the case of a known watch working over a long period (216 hours of independent operation), in which the barrels are coaxial and the springs disposed in series. The total torque working on the first pinion of the mechanism is equal to the torque of one of the springs.
With solution (c) the springs work in parallel and exert a torque 2M (the torques are added together) on the first member of a gear train. A watch is known in which this solution is applied, comprising two barrels, disposed on the same plane, which simultaneously engage the center-wheel. The considerable energy available makes it possible to maintain the movement of a high frequency spring balance oscillator (more than 30,000 alternations per hour), thus improving the performance of the watch without diminishing the working reserve of the watch (50 h). However, the high torque (of the order of 1500-2000 gr.mm) created by the combined action of the two driving springs implies considerable forces which result in deterioration of the conditions of engagement and pivoting of the components of the mechanism that a total yield of the gear-train mechanism is reduced, since part of the available energy is dissipated in overcoming friction.
In order to improve to some extent the yield of the mechanism of this watch, the ratio of the gearing between each of the barrels and the centre pinion has been modified (1:5 instead of 1:7). The barrels then turn faster (30% faster than previously); the torque is reduced at the same ratio and, all things otherwise remaining equal, the yield is slightly improved.
This prior art may still be considerably improved, and this is the object of the invention.